In macaques, visuomotor neurons in and around MIP and V6A have been
found to respond more during planned reaches than during planned
saccades to visual targets. Recent fMRI studies have attempted to find
a human homologue of this area. However, in those studies, (1)
pointing, instead of reaching, was compared to saccades, (2) delayed
(planned), rather than immediately executed hand movements were used,
and (3) the effect of seeing the hand was not investigated. It remains
unclear whether naturalistic reaching activates human medial parietal
areas more than saccades, whether that activation is present during
execution of reaching, and whether the activation depends on whether
the reaching hand is visible. Also, it is unknown if observation and
motor imagery of reaching activate those medial parietal areas. Here,
we addressed these questions with fMRI and cortical-surface-based
methods. In experiment 1, subjects either fixated centrally, or
reached or saccaded to peripheral visual targets. In experiment 2,
subjects reached on a visually hidden horizontal plate by moving their
hand from the center to peripheral locations corresponding to visual
target locations (e.g. targets at "12 o'clock" corresponded to an
upward movement on the plate) and made saccades as before. Both
visually guided and non-visually guided reaching activated a medial
parietal area more strongly than saccades. Saccade activations
partially overlapped with the medial region activated by reaching, but
were generally less strong. In experiment 3, subjects either observed,
imagined, or executed reaching to targets presented on a screen.
Interestingly, observed and imagined reaching activated the same
medial parietal region identified above by comparing reaching with
saccades. These findings suggest the presence of a medial parietal
human homologue to monkey MIP/V6A that is activated more by reaching
than by saccades, and that may contain neurons involved in execution,
mental simulation, and observation of action.